4 M. Martarelli et al. Fig. 1.3 Villa Greppi’s painted loft. (a) Frescos on camorcanna. (b) Damaged camorcanna without mortar and painting For these reasons, vibration measurements were performed before and after the restoration following two approaches: 1. The vibration response measurement at local level, with a dense spatial sampling on each panel (where the fresco was present), that can be considered a panel level investigation. 2. The vibration response measurement at global level, for the entire vault where one measurement point for each panel was taken, that can be considered a ceiling level investigation. The vibration response of the structure was measured in both cases in terms of velocity by means of an LDV that gives mobility Frequency Response Function (FRF). The measurement was performed on the frescoes surface in the vertical direction with a frequency range of 0–512 Hz and a frequency resolution of 312.5 mHz. Measurement points were selected to highlight the local and the global behavior respectively. The panel analysis was performed on all ceiling coffers except to the number 11, 12 and 51, see scheme in Fig. 1.2b, since the frescoes were missing on those panels. Each coffer was forced into vibration by means of an electrodynamic shaker operating as an inertial vibrodyne. As shown in Fig. 1.4b, the shaker was leaning on the vault structural beam of the ceiling and it was pushing an inertial mass of 500 g. The reaction force was acting as excitation of the structure. A load cell was measuring the force applied on the mass, which corresponded to the sum of dynamic forces applied, as a reaction force, by the shaker on the beam. The panel vibration response was measured by the LDV pointing on a regular grid of points on the fresco surface. A grid of 6 ×7 points was sat for the square panel and 8 ×7 points grid for rectangular one. The measured data set is made of FRF functions, obtained as ratio between the vibration velocity response of the panel and the force imparted in input by the electrodynamic shaker. Each FRF is weighted by the transmission path from the shaker to each measurement point and indicates how the energy travels through the different supporting elements (beams, panel, fresco). Therefore, diagnostics is possible: loss of signal amplitude could be due to a structural problem, e.g. fresco detachment. The global behavior of the ceiling has been estimated by performing an additional test. Excitation was provided by an instrumented hammer on the ceiling beam while response was measured by LDV on the central point of each panel. The vibration velocity was measured by the LDV that was sequentially moved at the central position of all the panels.
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